Background: Each year, people spend an estimated 970 million person-days at marine and freshwater beaches in the United States. Numerous studies have measured swimming-related health risks associated with fecal indicator bacteria exposure in the general population (all ages), but individual studies usually do not enroll enough swimmers to estimate health risks separately among young children. Children are hypothesized to have a higher risk of swimming-related illness than adult swimmers because they spend more time in the water, are more likely to swallow water (the primary mechanism of pathogen exposure), and have less developed immune systems. However, swimming-associated risks and health burdens among young children represent a key knowledge gap in the field: individual studies have been too small to look at this age group. Between 2003 and 2009 our teams completed longitudinal cohort studies at 13 marine and freshwater beaches in the United States. We now have a unique opportunity to combine these data to fill this knowledge gap. Aims: We have two Specific Aims that we will achieve with secondary analyses using the combined, 13-beach dataset. The analysis in Aim 1 will characterize child exposure and will use water exposure and swimmer outcome data from the 13 beaches to estimate the association between Enterococcus levels and gastrointestinal illness among children 10 years old. The analysis in Aim 2 will estimate the health burden among children 10 years old that is attributable to swimming in water with Enterococcus levels that exceed national beach regulatory guidelines. This will be the first large-scale study to characterize exposure, health risk, and health burden from swimming in contaminated water among young children in the United States. Methods: This study will combine data from 13 prospective beach cohort studies conducted by this proposal team that used highly comparable study designs, water sampling procedures, laboratory analysis methodology, and health outcome measurement. The combined dataset will include 74,531 individuals (49,267 swimmers) of all ages and 16,022 children (13,499 swimmers) 10 years old. All studies measured Enterococcus (an indicator bacteria of fecal contamination) using both qPCR and culture-based methods, and gastrointestinal (GI) illness in swimmers 10-14 days following the beach visit. In Aim 1, we will characterize child exposure and estimate the association between Enterococcus levels and subsequent GI illness among swimmers 10 years old using continuous measures of Enterococcus and dichotomous measures which classify days by whether or not water conditions exceeded national guidelines. In Aim 2, we will estimate the health burden of GI illness among children 10 years old that is attributable to swimming in water exceeding national regulatory guidelines for Enterococcus (> 35 colony forming units / 100 ml, measured by EPA 1600). We will estimate attributable risk using a population intervention model parameter, which is a significant advance over prior work in this field. Project Public Health Relevance: Each year in the United States, tens of millions of people swim at fresh- and marine water beaches impacted by fecal contamination. Epidemiologic studies consistently show a strong association between bacteria levels in the water and the risk of gastrointestinal illness (e.g., diarrhea), but individual studies are not typically large enough to estimate the risk separately for young children despite some evidence of higher risk for these children. This analysis will generate the first comprehensive estimates of the exposure, health risk, and health burden associated with swimming in contaminated water for children in the United States by combining data from 13 recent beach studies (13,499 swimmers 10 years old).